Wheel stability control based on the moment of an electrical motor

ABSTRACT

A wheel stability control system for an electric vehicle including an electric motor, a drive inverter, and an electronic control unit (ECU) including a computer readable, non-transitory memory (memory) and an electronic processing unit (EPU). The memory stores information including an optimal acceleration and deceleration curve and the electrical characteristics of the electric motor. The EPU calculates the electrical moment of the electric motor from inputs from the drive inverter and the electrical characteristics of the electric motor. The ECU compares the electrical moment and the angular speed of the motor with the optimal acceleration and deceleration curve, and if the acceleration or deceleration of the electric motor is out of a predetermined range when compared to the optimal acceleration and the optimal deceleration, it reduces the electrical moment applied by the electric motor.

BACKGROUND

In electric vehicles, as in traditional vehicles, maintaining controlover the angular speed of the wheels is critical to maintaining tractionwith the road surface. A loss of fraction can occur from excessiveacceleration or deceleration (i.e. hard braking). When accelerating, thewheels can receive excessive torque from the electric motor. This causesa loss of traction with the road surface and wheel spin. Similarly, whenthe wheels receive excessive braking force, the wheels lose tractionwith the road surface. Modern vehicle systems strive to eliminate a lossof traction and wheel spin with wheel stability control systems. Thesesystems include, for example, antilock braking, traction control, andstability control.

Wheel stability control systems are limited in effectiveness by theability of the vehicle's sensors to determine the vehicle's behavior. Insome situations, the vehicle's sensors provide inaccurate or misleadinginformation about the vehicle's behavior. This situation can arise evenwhen the sensors are performing their function correctly and accurately.For example, a wheel angular speed sensor may be correctly detecting thewheel's angular speed, but if the wheel is not maintaining traction withthe road surface, the information inaccurately describes the vehicle'sspeed. Inaccurate information about the vehicle can cause the wheelstability control systems to underperform. Therefore, systems andmethods for gathering accurate information about a vehicle and usingthis information in modern control systems are highly desired.

SUMMARY

In one embodiment, the invention provides a wheel stability controlsystem for optimizing the acceleration and deceleration of an electricvehicle while maintaining wheel stability. A power system including anelectric motor and a drive inverter determines the angular speed, theinput current, the input voltage, and a phase angle between the inputcurrent and the input voltage for the electric motor. An electroniccontrol unit (ECU) calculates an electrical moment of the electric motorby using the input current, the input voltage, the phase angle, and insome embodiments, the electrical characteristics of the electric motor.The electrical moment of the electric motor is the mechanical torque onthe electric motor that is created by the active power delivered to theelectric motor. Additional mechanical torque is applied by the hydraulicbraking system.

The motor's angular speed is directly related to the wheel's angularspeed, and thus, with a simple calculation, the motor's angular speedcan provide an accurate value of the wheel's angular speed. In normaldriving conditions (i.e. very low tire slip), the electronic controlunit can determine the speed of the vehicle (i.e. the tangential speedof the wheels) from the angular speed of the motor's angular speed.Therefore, the motor's angular speed provides a way to determine thevehicle's speed without a wheel angular speed sensor. Additionally, themotor's acceleration is determined from the motor's angular speed.

The electronic control unit is preloaded with information about thevehicle's performance. The vehicle's performance can be determined bytesting, which can reveal the optimal acceleration and decelerationvalues for that vehicle type. The optimal acceleration and decelerationvalues can be plotted on a curve and preloaded into the electroniccontrol unit. Based on the optimal values, the electronic control unitdetermines an optimal acceleration or deceleration for the electricmotor for the motor's current angular speed. The ECU compares theoptimal acceleration or deceleration for the electric motor to thecalculated acceleration or deceleration for the electric motor. The ECUadjusts the active power to the electric motor by adjusting the inputcurrent or input voltage so that the electrical moment is varied, thusvarying the motor's acceleration or deceleration. The ECU matches themotor's acceleration and deceleration with the optimal acceleration ordeceleration to achieve high performance while maintaining safety.

In another embodiment, the invention provides a method of controllingwheel stability of an electric vehicle including determining electricalcharacteristics, an input current, an input voltage, a phase anglebetween the input voltage and the input current, and an angular speed ofthe electric motor. The information is stored in the ECU, and the ECUcalculates an electrical moment of the electric motor based on the inputcurrent, the input voltage, the phase angle, the angular speed, and insome embodiments, the electrical characteristics of the electric motor.The acceleration or deceleration of the electric motor is calculatedbased on the angular speed of the electric motor. Optimal values foracceleration and deceleration are compared to the actual accelerationand deceleration, and when the optimal values are exceeded, theelectrical moment is adjusted to bring the actual acceleration anddeceleration to the optimal acceleration and deceleration values.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a power system for an electric vehicle.

FIG. 2 illustrates an electronic control unit and communication system.

FIG. 3 is a graph that illustrates an optimal acceleration anddeceleration curve.

FIG. 4 illustrates an embodiment containing a differential and two drivewheels.

FIGS. 5A and 5B illustrate an electrical moment over time for twodifferent braking configurations.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

FIG. 1 illustrates an embodiment of a drive system for an electricvehicle. A battery 100 is electrically coupled to a drive inverter 102,which is electrically coupled to an electric motor 104. The electricmotor 104 includes a motor shaft 106, which is mechanically coupled to adrive wheel 108. An electronic control unit 110 (ECU) is connected tothe battery 100, the drive inverter 102, and the electric motor 104. Insome embodiments, the motor shaft 106 is also a drive shaft, and inother embodiments, the motor shaft 106 is coupled to a drive shaftthrough gearing. In FIG. 1, the motor shaft 106 is coupled to a drivewheel 108. The electric motor 104 can be any type of electrical motorthat has sufficient horsepower to drive the vehicle, including, forexample, a synchronous motor. A power system of the vehicle includes thedrive inverter 102 and the electric motor 104. The power systemcommunicates with the ECU 110 and provides information about theelectric motor's 104 input current, input voltage, phase angle betweenthe input current and the input voltage, angular speed, and in someembodiments, the active power.

During acceleration, the battery 100 provides power to the driveinverter 102 by supplying the drive inverter 102 with direct current(DC). The drive inverter 102 converts DC into alternating current (AC)for use in the electric motor 104. The electric motor 104 convertselectric power into mechanical power by driving the motor shaft 106. TheECU 110 communicates with the drive inverter 102 and the battery 100 tomanage the power delivery to and from the electric motor 104. Duringbraking, the ECU 110 controls the power conversion process from thevehicle's momentum to the battery 100. The braking system 216decelerates the vehicle both through an electrical moment provided bythe electric motor 104, and by hydraulic braking. Preferably, theelectric motor 104 provides most or all of the braking moment, becausethe electric motor 104 and drive inverter 102 recapture some of thevehicle's momentum and convert it back into electrical energy forstorage in the battery 100.

FIG. 2 illustrates a control and communication system for an electricvehicle. The ECU 110 includes an electronic processing unit 202 (EPU), acomputer readable, non-transitory memory 204 (memory), and aninput/output interface 206. The ECU 110 can comprise multiple andseparate components including, for example, a vehicle stability module,an engine control module, a drive control module, and a domain controlunit. The memory 204 stores information and programs for use by theelectronic processing unit 202. The electronic processing unit 202 isconnected to the memory 204 and the input/output interface 206. Theinput/output interface 206 sends and receives data from the vehicle'ssensors. The data is transmitted on a vehicle communication bus 208(e.g. a CAN bus or a FlexRay bus) or can be communicated through adirect connection to the ECU 110. In some embodiments, a wheel angularspeed sensor 210 and a vehicle acceleration sensor 214 provide the ECU110 with information about the vehicle. The ECU 110 is also connected tothe braking system 216. Other information such as the input current, theinput voltage, and the angular speed of the electric motor 104 are sentdirectly to the ECU 110 by the drive inverter 102. The embodiment inFIG. 1 is free from a wheel angular speed sensor 210, because theangular speed of the drive wheel 108 is calculated from the angularspeed of the electric motor 104. However, the wheel angular speed sensor210 is useful in embodiments where one electric motor 104 powersmultiple drive wheels. The vehicle acceleration sensor 214 measures theacceleration of the vehicle in the longitudinal direction of travel(i.e., the ax direction). A road inclination is determined using thevehicle acceleration sensor 214 by determining the acceleration due tothe road inclination. The road inclination can also be determined byusing other sensors and methods.

The memory 204 contains information about the electrical characteristicsof the electric motor 104, which the electronic processing unit 202 usesto calculate the electrical moment from the signals from the driveinverter 102. The electrical moment can be estimated for an asynchronousmotor as follows:

${{Electrical}\mspace{14mu}{Moment}} \approx {C*\frac{P_{electric}}{n}}$

In this equation, C represents the motor specific constant, P_(electric)represents the active power, and n represents the angular speed of themotor. For a three-phase synchronous motor, the electrical moment can becalculated as follows:Electrical moment≈P _(electric)P _(electric)=√{square root over (3)}*U*I*cos φ

In these equations, P_(electric) represents the active power to theelectric motor 104, U is the voltage of the electric motor 104, I is thecurrent of the electric motor 104, φ is the angle between U and I, and fis the frequency of the electric motor 104. The ECU 110 calculates theelectrical moment by using the equations shown above. Additionally, theECU 110 determines the acceleration or deceleration of the electricmotor 104 from the electrical moment and the angular speed of theelectric motor 104 by calculating the derivative of the angular speed.The ECU 110 can increase the accuracy of these calculations by includingadditional terms to compensate for nonlinear effects (e.g., thermaleffects or friction).

The memory 204 also contains information about the optimal accelerationand deceleration curve 300 for the vehicle as shown in FIG. 3. The ECU110 uses the optimal acceleration and deceleration curve 300 to comparethe electrical moment and the angular speed of the electric motor 104 tothe optimal values, and the ECU 110 adjusts the drive inverter 102 andthe electric motor 104 to match the optimal acceleration anddeceleration curve 300.

The optimal acceleration and deceleration curve 300 shown in FIG. 3allows for calculation of optimal acceleration and deceleration based ona predetermined range. Each type of vehicle has a maximum accelerationand deceleration depending on the vehicles characteristics (e.g. weight,horsepower, brakes, and tires). The optimal acceleration anddeceleration curve 300 depicts the maximum reliable acceleration anddeceleration for a given vehicle and for a given value of an optimalslip. The graph in FIG. 3 illustrates the acceleration curve and thedeceleration curve on the same line; however, these curves can bedistinct and used independently. The horizontal axis represents anelectrical moment applied by the electric motor 104 as described above.The electrical moment is positive and increases in strength towards theright. On the left side of FIG. 3, the electrical moment is applied inthe opposite direction and increases in strength toward the left. Theleft side illustrates an electrical moment for braking. The verticalaxis on the graph represents acceleration of the drive wheel 108 in theforward direction. As the electric motor 104 increases the electricalmoment on the electric motor 104, the acceleration and the speed of thedrive wheel 108 increases. If the electric motor 104 createsacceleration of the electric motor 104 greater than the optimalacceleration, the drive wheel 108 will lose traction and increase thetire slip. This is shown by the upper breakaway point 302. The curvedportion of the line that extends from the upper breakaway point 302represents increasing slip of the drive wheel 108 as the electricalmoment increases. The upper breakaway point 302 can occur anywhere alongthe acceleration curve. If the vehicle hits the upper breakaway point302 and thereby enters the upper area of instability 306, the vehiclebecomes unstable. Conversely, the area below the optimal accelerationcurve represents typical driving situations where maximum accelerationis not needed.

During braking, if the electric motor 104 creates an electrical momentgreater than the optimal deceleration, the drive wheel 108 will losetraction and slip. This is shown by the lower breakaway point 304. Thecurved portion of the line that extends from the lower breakaway point304 represents increasing slip of the drive wheel 108 as the electricalmoment increases in strength. The lower breakaway point 304 can occuranywhere along the deceleration curve. If the vehicle hits the lowerbreakaway point 304 and thereby enters the lower area of instability308, the vehicle becomes unstable. This occurs when an excessive brakingmoment is applied. The area above the optimal deceleration curve depictsa region of slower deceleration of a drive wheel 108 and thus, normalbraking.

The actual acceleration and deceleration of an electric vehicle arecontrolled by the electrical moment and the angular speed delivered bythe electric motor 104. As stated above, excessive electrical momentcauses a loss of traction with the road surface. During operation of thevehicle, the upper breakaway point 302 and the lower breakaway point 304are prevented from occurring by the ECU 110. The ECU 110 limits theelectric motor 104 to acceleration of the drive wheel 108 just below theacceleration represented by the upper breakaway point 302. Similarly,the ECU 110 limits the electric motor 104 to deceleration of the drivewheel 108 to just below the deceleration represented by the lowerbreakaway point 304. This achieves high performance acceleration anddeceleration while maintaining good traction with the road surface.

The optimal acceleration and deceleration curve 300 is stored in thememory 204 of the ECU 110. The optimal acceleration and decelerationcurve 300 can be preloaded into the ECU 110 before delivery of thevehicle to customers. As described above, the ECU 110 uses the optimalacceleration and deceleration curve 300 to control the stability of thevehicle. The ECU 110 accomplishes this by comparing the angular speed ofthe electric motor 104 to an optimal angular speed based on the optimalacceleration and deceleration curve 300. When accelerating, if theangular speed of the electric motor 104 is greater than the optimalangular speed as calculated by the electronic processing unit 202, theECU 110 reduces power to the electric motor 104. When decelerating, ifthe angular speed of the electric motor 104 is less than the optimalangular speed, the ECU 110 reduces the electrical moment for brakingthat is generated by the electric motor 104. The ECU 110 performs thisprocess continuously as the vehicle is in motion. This allows the ECU110 to closely match the angular speed of the electric motor 104 to theoptimal angular speed as determined by the optimal acceleration anddeceleration curve 300.

FIG. 4 illustrates an embodiment with drive wheels 108 a and 108 b. Inthis embodiment, the motor shaft 106 couples to a differential 406. Thedifferential 406 transmits power to the drive wheels 108 a and 108 b,which allows the drive wheels 108 a and 108 b to have different angularspeeds. This embodiment includes a plurality of wheel angular speedsensors 210 a and 210 b, located proximal to each drive wheel, whichmeasure the angular speed of each of the drive wheels 108 a and 108 b.The angular speed of each of the drive wheels 108 a and 108 b istransmitted to the ECU 110, which compares the angular speed of each ofthe drive wheels 108 a and 108 b with the optimal angular speed of theelectric motor 104. Since the differential 406 applies the electricalmoment and the angular speed of the electric motor 104 in known ratios,the ECU 110 determines a range of speeds for the drive wheels 108 a and108 b. Based on the measured angular speeds of the drive wheels 108 aand 108 b, the ECU 110 determines the tire slip of the drive wheels 108a and 108 b. The ECU 110 uses the tire slip in performing the electronicstability control as described herein. Other embodiments include drivewheels 108 a and 108 b that are independently driven by multipleelectric motors.

FIGS. 5A and 5B are illustrations of the application of the electronicstability control during deceleration. On the vertical axis, the momenton the drive wheel 108 from the braking system 216 is shown. Time isshown on the horizontal axis. FIG. 5A illustrates an electriccooperation braking mode 500. This is a mode where the hydraulic brakesand the electric motor 104 work in cooperation to slow the vehicle. FIG.5A demonstrates the effect of the invention for electric cooperationbraking mode 500 at optimal deceleration. The ECU 110 applies amodulating electrical moment 504A as the angular speed of the electricmotor 104 approaches and meets the optimal angular speed as determinedby the optimal deceleration curve. FIG. 5B illustrates an electriccompensation braking mode 502. In this mode, the ECU 110 applies amodulating electrical moment 504B to compensate for a disproportionatehydraulic brake torque. The modulating electrical moment 504B isillustrated as maintaining an effective electrical moment 508 of theelectric motor 104 just within the stability region.

Thus, the invention provides, among other things, a wheel stabilitycontrol system for controlling the stability of a vehicle based on anoptimal acceleration and deceleration curve 300 and a determination ofan electrical moment of an electric motor 104. Various features andadvantages of the invention are set forth in the following claims.

What is claimed is:
 1. A wheel stability control system for an electricvehicle, the system comprising: a power system comprising an electricmotor having electrical characteristics, the power system configured todetermine an angular speed, an input current, an input voltage, and aphase angle between the input current and the input voltage to theelectric motor; an electronic control unit (ECU) including a computerreadable, non-transitory memory, the memory storing informationincluding an optimal acceleration and deceleration curve of the vehicleand the electrical characteristics of the electric motor, and anelectronic processing unit (EPU), wherein the ECU is configured to:calculate an electrical moment of the electric motor based on theangular speed, the input current, the input voltage, and the phase angleof the electric motor provided by the power system and on the electricalcharacteristics of the electric motor, determine the acceleration ordeceleration of the electric motor from the angular speed of theelectric motor, calculate the optimal acceleration and the optimaldeceleration of the electric motor based on the angular speed of theelectric motor and based on the optimal acceleration and decelerationcurve, determine if the acceleration or the deceleration of the electricmotor exceeds the optimal acceleration or the optimal deceleration ofthe electric motor, and when the acceleration or the deceleration of theelectric motor exceeds the optimal acceleration or the optimaldeceleration, reduce the electrical moment of the electric motor untilthe acceleration or the deceleration of the electric motor matches theoptimal acceleration or the optimal deceleration.
 2. The system of claim1, wherein the ECU configured to determine if the acceleration or thedeceleration of the electric motor exceeds the optimal acceleration orthe optimal deceleration of the electric motor further comprisesconfiguring the ECU to: determine whether the angular speed of theelectric motor exceeds a predetermined range when accelerating, whereinan optimal acceleration curve determines the predetermined range, anddetermine whether the angular speed of the electric motor is below apredetermined range when decelerating, wherein an optimal decelerationcurve determines the predetermined range.
 3. The system of claim 1,wherein the wheel stability control system is free from a wheel angularspeed sensor.
 4. The system of claim 1, further comprising anacceleration sensor that determines an acceleration of the vehicle andprovides a value of the acceleration of the vehicle to the ECU, whereinthe ECU adjusts the optimal acceleration and deceleration curve inresponse to the information from the acceleration sensor.
 5. The systemof claim 1, wherein the electric motor comprises a synchronous motor. 6.The system of claim 1, wherein the optimal acceleration and decelerationcurves are determined and preloaded into the memory prior to delivery ofthe vehicle to a customer.
 7. The system of claim 1, further comprisinga second electric motor, wherein the electric motor is coupled to andpowers a first drive wheel and the second electric motor is coupled toand powers a second drive wheel.
 8. The system of claim 1, furthercomprising: a differential, wherein the differential transmits powerfrom the electric motor to a first drive wheel and to a second drivewheel; and a first wheel angular speed sensor located proximal to thefirst drive wheel and a second wheel angular speed sensor locatedproximal to the second drive wheel, wherein the first wheel angularspeed sensor and the second wheel angular speed sensor transmitinformation to the ECU, and wherein the ECU determines a tire slip ofthe first drive wheel and the second drive wheel based on the firstwheel angular speed sensor and the second wheel angular speed sensor. 9.The system of claim 1, wherein the power system includes a driveinverter configured to determine the angular speed, the input current,the input voltage, and the phase angle between the input current and theinput voltage to the electric motor.
 10. A method for controlling drivewheel stability in a vehicle powered by an electric motor, the methodcomprising: determining electrical characteristics of the electricmotor; determining an input current, an input voltage, a phase anglebetween the input voltage and the input current, and an angular speed ofthe electric motor; and storing information about the input current, theinput voltage, the phase angle, the angular speed, and the electricalcharacteristics of the electric motor into a computer readable,non-transitory memory for use by an electronic control unit (ECU) tocalculate an electrical moment of the electric motor based on the inputcurrent, the input voltage, the phase angle, the angular speed, and theelectrical characteristics of the electric motor, calculate anacceleration or a deceleration of the electric motor based on theangular speed of the electric motor, determine an optimal accelerationor optimal deceleration of the electric motor based on an optimalacceleration and deceleration curve and the angular speed of theelectric motor, compare the acceleration or deceleration of the electricmotor to the optimal acceleration or optimal deceleration, and when theacceleration or the deceleration of the electric motor exceeds theoptimal acceleration or the optimal deceleration, adjust the electricalmoment of the electric motor to bring the acceleration or thedeceleration to the optimal acceleration or the optimal deceleration.11. The method of claim 10, further comprising: determining whether theangular speed of the electric motor exceeds a predetermined range whenaccelerating, wherein an optimal acceleration curve determines thepredetermined range, and determining whether the angular speed of theelectric motor is below a predetermined range when decelerating, whereinan optimal deceleration curve determines the predetermined range. 12.The method of claim 10, wherein the method is free from a wheel angularspeed sensor.
 13. The method of claim 10, further comprising the stepsof: determining an acceleration of the vehicle by using a vehicleacceleration sensor and transmitting information from the vehicleacceleration sensor to the ECU, wherein the ECU adjusts the optimalacceleration and deceleration curve in response to the vehicleacceleration sensor.
 14. The method of claim 10, wherein the electricmotor is a synchronous motor.
 15. The method of claim 10, wherein theoptimal acceleration and deceleration curve is determined and preloadedinto a computer readable, non-transitory memory prior to delivery of thevehicle to a customer.
 16. The method of claim 10, further comprising:transferring power from the electric motor via a differential to twodrive wheels; determining an angular speed for each of the two drivewheels by using a wheel angular speed sensor located proximal to each ofthe two drive wheels; and transmitting information from each wheelangular speed sensor to the ECU, wherein the ECU determines a tire slipof each of the two drive wheels.